Planetary gear boxes are known. FIG. 1 shows a sectional view of a known planetary gear box arrangement providing speed reduction between two co-axially aligned shafts 2, 6. The gear box comprises a sun gear 1 centred on a centreline C-C. The sun gear 1 is mounted to an input shaft 2 which is mounted for rotation in a first bearing 3. The sun gear 1 meshes with an array of planet gears 4 (only one of which is shown) arranged around the toothed circumference of the sun gear 1. A ring gear 5 encircles and meshes with the planet gears 4. The ring gear 5 is fixed in position and so, upon rotation of the sun gear 1, the planetary gears 4 are forced to rotate around their individual axes A-A and travel within the ring gear 4 and around the sun gear 1.
The planet gears 4 are each mounted for rotation in a planet carrier 7 which in turn connects to an arm 6a of an output carrier 6b via a spherical bearing 7a. A bearing pin 10 locates the bearing 7a in a bore of the planet carrier 7. The arm 6a is arranged to transmit the output via an output shaft 6 which nominally shares a common axis C-C with the input shaft 2. The output shaft 6 is further mounted for rotation with respect to a static casing by means of a bearing or pivot 9. The planet carrier 7 is arranged for rotation about the axis C-C by means of a bearing 8. The planet carrier 7 and the output shaft 6 rotate together.
It will be appreciated that the bearing pin 10 between the planet carrier 7 and a planet gear 4 in high capacity power gearboxes is loaded due to torque and centrifugal effects. The connection between the planet carrier 7 and bearing pin 10 must transmit the full torque of the gearbox, as well as centrifugal loads arising due to the planet gear orbiting. At each bearing pin 10, these loads can be comparable to the thrust of a jet engine. Consequently, unhelpful deflections occur between these components which are very challenging to control.
It is known to use interference fits between the planet carrier 7 and bearing pin 10. However, with every increase in scale of the gear box, assembly by interference fit becomes a greater challenge. If insufficient preload/fit is used, the join between the pin 10 and planet carrier 7 can open up allowing the planet gear 4 to move radially under centrifugal loading, taking it closer to the ring gear 5 and further from the sun gear 1. Consequences may include gear overstressing, excessive gear noise, or weakening of gear teeth due to thinning. Other side effects such as fretting can also cause operational issues if too much movement occurs at this interface.
In alternative arrangements, the sun gear may be fixed in position and the ring gear rotated to drive the planet gears.
Outside of gear box design, it is known to use tapered bores to provide improved interference fits between concentric shafts. For example, a tapered journal and bore may be driven directly together in a “taper-taper” arrangement. In other arrangements, a tapered sleeve is driven between a tapered surface and a parallel surface. Such joins may be assembled “dry” or “wet”. In a dry assembly, reliance is entirely upon an axial load to engage the components. In a wet assembly, a hydraulic fluid (typically oil) may be injected under very high pressure into a space between the components. The fluid serves to separate and lubricate the surfaces during tightening and extraction. The fluid is typically supplied through a gallery which is part of the tapered sleeve.